Clean up multi byte nop support on x64 to more closely match IA32.

src/x64/assembler-x64.h

Index: src/x64/assembler-x64.h
===================================================================
--- src/x64/assembler-x64.h	(revision 10142)
+++ src/x64/assembler-x64.h	(working copy)
@@ -636,6 +636,7 @@
   // possible to align the pc offset to a multiple
   // of m, where m must be a power of 2.
   void Align(int m);
+  void Nop(int bytes = 1);
   // Aligns code to something that's optimal for a jump target for the platform.
   void CodeTargetAlign();
 
@@ -1154,7 +1155,6 @@
   void hlt();
   void int3();
   void nop();
-  void nop(int n);
   void rdtsc();
   void ret(int imm16);
   void setcc(Condition cc, Register reg);
@@ -1398,7 +1398,7 @@
     return static_cast<int>(reloc_info_writer.pos() - pc_);
   }
 
-  static bool IsNop(Address addr) { return *addr == 0x90; }
+  static bool IsNop(Address addr);
 
   // Avoid overflows for displacements etc.
   static const int kMaximalBufferSize = 512*MB;